Tropical Depression 7 in the Caribbean – a Potential Threat

Tropical Depression 7 forms in the Northeast Caribbean. GOES Floater RGB imagery of the depression shortly after its first advisory by the National Hurricane Center. Note proximity to Puerto Rico where tropical storm warnings are in effect.

Two tropical waves interacting in the northeastern Caribbean have produced an area that organized just enough in low level wind structure for the National Hurricane Center to designate the area Tropical Depression 7 (TD7). This system is currently impacting Puerto Rico and the leeward islands with squally showers and potentially flooding rain. It could also spread towards the Dominican Republic as the depression exits the Caribbean. Tropical Storm watches and warnings are in effect for those mentioned islands and countries.

Meanwhile, a slow moving, weakening cold front to the west of Bermuda and a pre-frontal trough of low pressure will be a major steering influence on this system. Timing is everything as usual with tracking tropical cyclones – exactly when, if and how a front interacts with the tropical cyclone will determine where it tracks. The current forecast track from the National Hurricane Center shows TD7 passing to the east of Bermuda late on Monday as it moves in the southwesterly flow ahead of the front stalled near Bermuda. It is important to note that this is a very tentative initial prediction and will likely change.

First Official Track for Tropical Depression 7 as appears on the Bermuda Weather Service Tropicals page. Valid as of 6pm September 4th Bermuda time.

This forecast is more uncertain than normal. For one reason, the front that is forecast to steer TD7 is already slow moving and might not make it as far east as forecast, or could push further east than forecast. But more importantly, TD7 formed during the interaction of two tropical waves. This left TD7 with an extension of low pressure to the northeast of its relatively poorly defined surface center – this is the remnants of the second tropical wave. As TD7 absorbs this wave, it will likely initially take a track more to the right of the official National Hurricane Center forecast track which seems to be based more on the general steering currents around the Bermuda-Azores High, ignoring a small scale feature like this. The merger between TD7 and the remnants of a tropical wave to its northeast may be a better defined storm with more spin and convergence available – but the process could slow initial strengthening because the storm is so fragile right now. Currently, the environment that TD7 is in appears somewhat favorable, at least enough for intensification into a tropical storm by tomorrow.

For Bermuda, expect some showers in the convergence ahead of this approaching frontal boundary and associated pre-frontal trough for today and tomorrow. But keep an eye on TD7 as it could be nearing Bermuda over the weekend. As the tropics can change quickly, it is important to stay tuned to the Bermuda Weather Service for the latest official advisory products and forecasts for Bermuda. The official forecast track for TD7 can be found at the National Hurricane Center, and a version with some Bermuda specific information can be found at the Tropicals section of the Bermuda Weather Service site.